This invention relates in general to a control valve and, in particular, to a control valve for controlling multiple function in a fluid operated system. More specifically, but without restriction to the particular use which is shown and described, this invention relates to a control valve especially adaptable for use in a steering and brake control system for a steering-by-driving vehicle. For convenience of illustration, the invention will be described with reference to its use in the steering and brake control system of a crawler tractor, but it is believed to have general utility in any steering-by-driving vehicle.
Control of the braking and steering systems for large steering-by-driving vehicles, such as crawler tractors, has been the concern of many engineers and numerous examples of systems and apparatus to accomplish this purpose have been disclosed in various prior art patents.
In U.S. Pat. No. 3,837,449, issued on Sept. 24, 1974 in the name of Gary A. Drone, one of the present inventors, there is described and claimed a braking valve system using mechanical biased brakes and hydraulic actuators controlled by a valve system. In the system disclosed therein, a decrease of transmission drive clutch fluid pressure, such as would be caused by shifting the vehicle transmission to neutral or by the dying of the engine, would automatically result in the application of the vehicle brakes to stop the vehicle.
In U.S. Pat. No. 3,765,454, issued on Oct. 16, 1973, in the name of David F. Carl, the other of the present inventors, a valve structure is disclosed wherein an axial slot in a valve spool modulates the pressure rise by providing a graduated rise in pressure in the working fluid being applied to a controlled member, such as a clutch. Such pressure modulation is of a particular utility in controlling the steering clutches of a steering-by-driving vehicle.
While G. A. Drone, U.S. Pat. No. 3,837,449, discloses a braking valve system which is highly satisfactory for a clutching and braking system, the braking system must be actuated separately from the clutching system by manually depressing the brake pedal, or automatically in response to conditions occurring within the vehicle transmission. However, there is no provision for actuating the vehicle braking system in response to the actuation of the vehicle steering clutches, as is desirable during many operations of steering-by-driving vehicles.
One attempt to provide a system wherein the vehicle brakes are actuable in response to operation of the vehicle steering clutches as well as independently actuable is disclosed in U.S. Pat. No. 3,895,703. This patent discloses a crawler tractor vehicle having steering clutches and associated steering brakes wherein the steering brakes may be actuated independently or without disengagement of the associated steering clutch. While this patent discloses a system wherein the vehicle brakes are independently operable as well as in response to actuation of the vehicle steering clutches, the system disclosed therein does not provide for automatic braking when the vehicle transmission is placed in neutral, nor does the system provide for fail-safe operation wherein the vehicle brakes are spring actuated and pressure released such that upon failure of the hydraulic fluid system, the brakes will automatically be engaged.